Electrical heating element



I n vznw'oq Zmw, ATroQnLys Basil Horafizlcl B HORSFIELD ELECTRICAL HEATING ELEMENT Flled July 24, 1940 Nov. 21, 1944.

Patented Noy.l21, 1 944 UNITED. STATES PAT-ENT OFFICE Basil Horsiield, Pittsburgh, Pa., assignor', by

mesne assignments, to Edwin L. Wierand Company, Pittsburgh, Pa., a corporation of Pennsylvania Application-Filly 24, 194p, Serial No. 347,185

- 9 Claims. (01. 20144) My invention relates to electrical heating elements and more particularly to such elements comprising a resistor disposed in and insulated from a metallic sheath. The principal object of my invention isto provide new and improved electrical heating elements and methods of making the same.

In the drawing accompanying and forming a part of this application, I have shown, for purposes of illustration, one form of electrical heating element of the aforesaid type, and in this drawing:

Figure 1 is a plan view ofan annular type of heating element, and

Figures 2 and 3 are sections taken on the lines 2-2 and 33 respectively of Figure 1.

The heating element illustrated comprises "a sheetmetal sheath in here shown as of annular form. The sheath II has disposed therein 'a resistor ll suitably insulated from the sheath by insulating material l2. The resistor is here shown as made of helically wound wire, but it will be apparent to those skilled in the art that it may be made of other forms of electric resistance material, wound or bent in other ways.

The sheath I is here shown as comprising a sheet metal channel I3 of annular form providing a plane wall it, and a sheet metal cover plate IS, the marginal portions 16 of the sides of the channel [3 being bent or crimped over the cover plate IS toolis then withdrawn, leaving the resistor ll an embossed portion 24 in the regionof the studs The ends 25, 26 of the resistor H are suitably connected to the studs l'l respectively, the intermediate helical portion of the resistor being dispatent to Wiegand 1,133,347, March 30, 1915, the tool, of course, having a resistor-supporting part which is of such shape and size that it will fit into the channel l3. The resistor ll, carried by the tool, is introduced into an open coverless channel, such 'as i3 before the margins ii are crimped, the channel B being partly filled to a predetermined level with insulating material which is in a plastic, plastoid, or impressionable condition, and consequently the resistor i I becomes embedded in the insulating material. The

embedded in the insulating material. Additional insulating material is then introduced in the channel, 'over the embedded resistor, and the in sulating material'may then be compressed to a desired density. The cover i5 and washers 22 being applied, the margins i6 are crimped over the cover, and the element subjected to high pressure to compact the insulating material i2. The element may be subjected to various drying or baking steps, or both, at various intermediate stages of manufacture before and after the cover [5 is applied, and as a final treatment if desired. l

The body of insulating material must of course be refractory, and it is desirable that it comprise a selected material that has suitable electrical-insulating properties, and desirably good heat-conducting properties. In instances where a bonding materialhas been mixed with the selected heat-conducting electrical-insulating material it has been common to use clay as the bonding material. My invention contemplates the avoidance of the use of clay.

I have found that a body of insulating material suitable for use in electrical heating elements may be made by mixing a selected electrical-insulating heat conducting material such as, for example, zircon (zirconium silicate) or other suitable material, with a bonding material made of magnesium'oxide and phosphoric acid. For example, the mixture for introduction into the sheath, as hereinbefore described, may be prepared as follows. The bonding material is prepared by mixing magnesium oxide (MgO) and phosphoric acid (HaPOO of strength,-both preferably chemically pure, in the proportion of magnesium pyrophosphate (MgzPrOr) and I mix the ingredients in the aforesaid proportions with the purpose of providing that compound. For reasons hereinafter stated, the mixture of magnesium oxide and phosphoric acid should be approximately 5% to 4%. The magnesium oxide and phosphoric acid are mixed to a thin slurry which is then dried down to a sticky putty-like mass. This massis then mixed with the zircon which desirably has been slightly dampened with water; and, preferably, tale is included in the mixture as a mechanical dilutant for the bonding material. This mixture is then used to embed the resistor as hereinbefore described.

' The amount of phosphoric acid and magnesium oxide are adjusted, to obtain slight acidity as hereinbeiore set forth, for the purpose of securing the desired plasticity and coherence, in other words, for securing the desired workability of the mass of insulating material. -It is desirable in connection with the making of electrical heating elements of the insulation-embedded resistor type that the mixture of selected heat-conducting electrical-insulating material and bonding mterial have about the mechanical workability of clay in the resistor-embedding stage of manufacture, the heating element being thereafter baked or otherwise heat treated to: convertffthe body of insulating material into a hard, firm cake.

The ingredients may be taken in the following proportions, as an example of a satisfactory batch: bonding material, comprising 260 grams of magnesium oxide mixed with 460 cubic centimetres of phosphoric acid of 85% strength, prepared as hereinbefore described, is mixed with 4000 grams of zircon and 180 grams of talc. This mass when baked provides a cake in the finished heating element of suitable firmness and hardness. Heating elements embodying such a body of insulating material when operating at energy dissipations up to approximately 30 watts per square inch of heating surface, show a remarkably low electrical leakage when the terminals of a source of alternating current are connected to the resistor and sheath respectively, in a range from 220 volts to a high test voltage of thcorder of 1200 volts.

While excellent results are obtained by using the ingredients for the body of insulating material in the proportions hereinbefore given, that is, about 14 to 15% of the bonding material, 81 to 82% of zircon, and 3 to 4% of talc, I do not limit myself to these specific proportions- Generally speaking, the percentage of bonding material is selected to provide a cake of satisfactory mechanical characteristics for a particular use while at the same time providing a cake having the desired electrical characteristics for that particular use. I prefer to include talc because the electrical characteristics are improved. The talc is used in such suiiicient percentage as will im prove the electrical characteristics of the cake but at the same time will not impair the.desired mechanical characteristics, that is, its desired hardness and firmness.

When tale is used I prefer to use an electrical insulator grade of talc or steatite. It is of course understood that talc when heated or burned is converted into steatite. The tale desirably should be low in lime and alkali salts, and should burn satisfactorily. A pure dead white talc which is white burning is preferred. A talc which containsa considerable percentage of magnetic material and iron oxide will burn a light brown and such a talc, while it may be used, is not as good as white-burning tale. The talc used in mixtures embodying my invention is desirably finely ground.

While I have'illustrated one form of electrical heating element of a type of heating elements, it will be understood that my invention may be embodied in other suitable forms ofthat type and in other suitable types of electrical heating elements.

From the foregoing it .will be apparent to those skilled in the art that each of the disclosed embodiments of my invention provides a new and improved electrical heating element, and accordingly, each accomplishes theprincipal object of my invention. Onthe other hand, it also will be. obvious to those skilled-the art that the disclosed embodiments of my invention may be variously changed and modified, or features thereof, singly or collectively, embodied in other combinations than those disclosed,.without departing from the spirit of my invention, or sacri flcing all of the advantages thereof, and that accordingly, the disclosure herein is illustrative only, and my inventionis not limited thereto.

I claim;

1. An embedded-resistor electric heating element comprising, am of embedding material consisting in major percentage of particles of selected refractory heat conducting electrical-insulating material, having said particles bonded with magnesium pyrophosphate.

2.'An embedded-resistor electric heating element comprising a body of embedding material consisting in major percentage'of particles of seconsisting in major percentage of particles of zircon, having said particles bonded with magzircon, having said particles bonded with mag-t nesium pyrophosphate.

5. An embedded-resistor electric heating element comprising a body of embedding material consisting in major percentage of particles of nesium pyrophosphate diluted with talc;

6. an embedded-resistor electric heating element comprising a body of embedding material consisting in major percentage of particles of zircon, having said particles bonded with magnesium pyrophosphate diluted with, tale in a per-.- centage appreciably to increase theresistance of said body to electrical leakage.

7. An embedded-resistor electric heating elel-y ment comprising a body of embedding material of; 1 which approximately is particles of selected 1 pyrophosphate to the percentage of said talc being of the order of approximately three to one to the order of approximately live to one.

9. An embedded-realismelectric heating element comprising a body at embedding material otwhichapproximately80% isparticles otzircon. having said particles bonded with magnesium pyrophosphate diluted with talc. the ratio of the percentage oi said magnesium orr ohosohate to the percentage 0! said talc being 0! the order of approximately three to one to the order of approximately ilve to one.

BASIL HORBFIEID. 

